The afferent connections to the primate superior colliculus have been investigated using horseradish-peroxidase histochemistry. Rhesus monkeys were prepared for chronic neurophysiological recording in order to localize the deep eye movement-related activity within the colliculus. Injections of horseradish peroxidase were made into these deep layers. After a survival period that permitted the enzyme to be transported from the terminal to the cell bodies of origin, the monkeys were sacrificed, perfused, and the tissue sectioned and reacted for peroxidase. Individual sections of the brainstem were examined for labeled cells. Preliminary experiments indicate that the major sources of afferent input to the primate colliculus arise from the substantia nigra (pars reticulata) and the parabigeminal nucleus. Other structures that project to the colliculus include the contralateral superior colliculus and the mesencephalic reticular formation. The presence of labeled cells in several other structures was variable, suggesting that labeling of these neurons may be due to spread of the injection site into extracollicular structures. Additional experiments will be needed to determine an inventory of brainstem structures that project to the colliculus and which may be providing saccade-related input to the deep layers.